The invention relates to an extrusion method in which at least one material feeding device is used to feed material through a pump unit into a nozzle.
The invention further relates to an extruder comprising at least one material feeding device, pump unit and nozzle arranged after the pump unit, the material feeding device being arranged to feed the material through a screw unit into the nozzle.
The invention also relates to a product produced with the method.
An extruder typically comprises a cylindrical stator provided with a long cylindrical rotor arranged inside the stator. A problem arises from how to achieve a uniform extruder yield and how to ensure the centricity and uniform radial thickness of the extruded product. To ensure the centricity of the product, the far end of the extruder usually has to be provided with a spider leg which causes weld lines in the product.
Prior art also knows a solution where the extruder comprises a conical rotor and a conical stator. This allows the extruder to be made relatively short, and the spider leg is not needed. An example of this solution is described in WO Patent Application 97/37832. The publication also teaches that the equipment comprises separately adjustable feeding devices for regulating the material flows to be fed to supply conduits arranged on different sides of the rotor. By adjusting the feeding devices and the speed of rotation of the rotor, the properties of the different layers in the extruded product are regulated. The solution in question allows a most versatile control of the product properties, but requires skill and expertise from the user.
U.S. Pat. No. 5,695,789 discloses equipment comprising at least two extruders and a gear pump arranged after each extruder, the gear pumps being used for feeding the material to be extruded into a nozzle. Each gear pump supplies material for a different product layer, the equipment thus producing a multilayer product. To control the thickness of the different layers, each gear pump is separately adjusted, in other words, the yield of each layer is controlled by adjusting the gear pump. The equipment is most complex, and the centricity and uniform radial thickness of the product cannot be sufficiently well ensured. In multilayer products in particular it is most difficult to achieve a good thickness profile in the separate layers and in the end product.
An object of the present invention is to provide a method and equipment allowing a product with a good centricity and thickness profile to be produced.
The method of the invention is wherein the pump unit is a radial pump unit comprising conical screws which are adjacently positioned and rotate in the same direction, the radial pump unit transferring an annular flow of material into the extruder""s nozzle.
Further, an extruder of the invention is wherein the pump unit is a radial pump unit comprising conical screws arranged to be adjacently positioned and to rotate in the same direction, the radial pump unit being arranged to transfer an annular flow of material into the extruder""s nozzle.
A product of the invention, in turn, is wherein the product comprises an outer helical orientation of a specific direction and an inner helical orientation of an opposite direction with respect to the outer helical orientation.
An essential idea of the invention is that the material feeding device supplies the material to be extruded in a melted form into the radial pump unit of the equipment, the radial pump unit comprising a plural number of conical screws which are adjacently positioned and rotate in the same direction. The radial pump unit transfers the material in an annular flow into the extruder""s nozzle. A preferred embodiment is based on the idea that the material feeding device is a melting screw. A second preferred embodiment is based on the idea that a continuous solid material, a liquid or gas, or a combination of these, is supplied through the screw. A third preferred embodiment is based on the idea that there is a plural number of material feeding devices and that the material to be extruded is supplied into the material feeding devices through a ring channel where their input ends are connected to. A fourth preferred embodiment is based on the idea that the equipment is provided with a plural number of radial pump units, which allows a plural number of material layers arranged one inside the other to be supplied into the extruder""s nozzle to produce multilayer products.
An advantage of the invention is that an extremely uniform yield is obtained using the equipment. In addition, it provides an extremely simple means for ensuring an excellent centricity of the product, thereby allowing products with extremely good centricity and radial thickness profile to be produced. Furthermore, an excellent thickness profile is achieved in multilayer products. The solution of the invention further allows the material or materials to be extruded to be most effectively melted and mixed, a most homogeneous material flow being thereby obtained. Further still, products produced by applying the solution of the invention can be reinforced in various different ways. A radial pump unit comprising screws that rotate in the same direction allows an extremely good mixing efficiency to be obtained, the output flow being helically oriented into opposite directions on the outer and inner surfaces of the extruded product.